Hydrocracking is a basic conversion process used in many petroleum refineries worldwide to reduce the molecular weight of petroleum derived feedstocks and convert the feedstock into more valuable products such as motor fuel, diesel fuel and lubricants. Hydrocracking also has other beneficial results such as removing sulfur and nitrogen from the feedstock by hydrodesulfurization and hydrodenitrification.
Feedstocks most often subjected to hydrocracking are gas oils and heavy gas oils recovered from crude oil by distillation. A typical heavy gas oil comprises a substantial portion of hydrocarbon components boiling above about 371° C. (700° F.), usually at least about 50% by weight boiling above 371° C. (700° F.). The typical vacuum gas oil normally has a boiling point range between about 316° C. (600° F.) and about 566° C. (1050° F.).
Hydrocracking is generally accomplished by contacting in a hydrocracking reaction vessel or zone the gas oil or other feedstock to be treated with a suitable hydrocracking catalyst under conditions of elevated temperature and pressure in the presence of hydrogen so as to yield a product containing a distribution of hydrocarbonaceous products desired by the refiner. The operating conditions and the hydrocracking catalysts within a hydrocracking reactor influence the yield of the hydrocracked products.
A general review and classification of different hydrocracking process flow schemes and a description of hydrocracking catalysts is provided in the book titled Hydrocracking Science and Technology, by Julius Scherzer and A. J. Gruia published in 1996 by Marcel Dekker, Inc., New York, ISBN 0-8247-9760-4, especially at pages 174-183. FIGS. 10.2, 10.3 and 10.4 show hydrotreating reactors upstream of the hydrocracking reactor. As noted therein it is an established practice to first pass a hydrocracking unit feed stream into a hydrotreating reactor in order to reduce the level of sulfur and nitrogen tied up in the target petroleum molecules. Two hydrocracking reaction zones may be used in series with some form of intermediate separation between the hydrocracking zones to reduce the amount of hydrogen sulfide and product hydrocarbons carried over to the second hydrocracking zone with the hydrocarbon phase. This type of unit is normally referred to a two stage hydrocracking unit as shown by FIGS. 10.4 and 10.5.
While the overall physical design of a hydrocracking process can be very important to the level of conversion and selectivity achieved by a hydrocracking process, these two measures of performance are always also tied to the abilities of the hydrocracking catalyst employed in the process.
Hydrocracking catalysts are subject to initial classification on the basis of the nature of the predominant cracking component of the catalyst. This classification divides hydrocracking catalysts into those based upon an amorphous cracking component such as silica-alumina and those based upon a zeolitic cracking component such as Y zeolite. Hydrocracking catalysts are also subject to classification on the basis of their intended predominant product of which the three main products are liquefied petroleum gas (LPG), naphtha, and “distillate”, a term which in the hydrocracking refining art refers to distillable petroleum derived fractions having a boiling point range which is above that of naphtha. Distillate typically includes the products recovered at a refinery as kerosene, diesel fuel, and aviation turbine fuel (ATF). LPG boiling range hydrocarbonaceous compounds typically contains C3 and C4 hydrocarbons, mostly propane and butane. The process disclosed herein relates to a zeolitic catalyst having improved selectivity for the production of distillate boiling range and LPG boiling range hydrocarbonaceous compounds. These catalysts normally comprise a zeolite component and a support or other component such as alumina or silica-alumina and a metal hydrogenation component.
It is known art to introduce nitrogen-containing streams, such as hydrocracking unit feedstocks or added ammonia, into the second stage of a two-stage hydrocracking unit to increase distillate selectivity. This and other changes in operating conditions allow the product distribution of an existing hydrocracking unit to be adjusted to a limited extent. However, the yield characteristics of the catalyst loaded in the unit are often of great importance in achieving a desired product distribution.
There is always a demand for new hydrocracking catalysts that provide lower costs, higher liquid product yields, and desired product streams. In some cases and in some parts of the world, it is highly desirable to produce maximum quantities of middle distillate including diesel and selective amounts of propane and butane for use as LPG. Processes for providing both of these two desirable product streams using the same or similar catalysts in the first and second stages of a two stage hydrocracking unit are sought.